The present invention concerns a liquid crystal display device having a liquid crystal layer sandwiched between substrates and a pair of polarizers disposed at outside of the liquid crystal layer, and having a structure in which the liquid crystal is oriented substantially in parallel to the surface of the substrate and twisted at an angle of from 120.degree. to 360.degree. in the thickness direction of the liquid crystal layer when the voltage is not applied.
The liquid crystal display mode mainly used so far is referred to as a twisted nematic (TM) type having a structure in which liquid crystal molecules are twisted by about 90.degree. between upper and lower substrates, and it utilizes the rotation of a polarization plane caused by the liquid crystal and elimination of such effect upon the application of voltage. The display mode is satisfactory in low time-division driving used for electronic digital clocks or pocket calculators but involves a drawback that the contrast is lowered or the visual angle is narrowed if it is driven at high time-division in order to increase the display capacity. The drawback is due to the fact that the ratio of the voltage applied on a selected point to the voltage applied on a nonselected point approaches 1 upon the high time-division driving.
In order to attain a display device of high contrast and wide visual angle, it is necessary to reduce the steepness .gamma. represented by the ratio (V.sub.50 /V.sub.10) of the voltage (V.sub.50) which changes the relative transmittance of the device by 50% to the voltage (V.sub.10) which changes the relative transmittance by 10%.
In the case of twisted nematic type, the .gamma. value is about 1.13. For reducing the .gamma. value, it has been proposed a method of increasing the twist angle of liquid crystal molecules and shifting the polarization axis from the orientation direction of liquid crystal, which is referred to as a SBE mode or STN mode. With such a method, the .gamma. value can be reduced to less than 1.1 and high time-division driving at about 1/400 duty is possible.
However, since such a method utilizes the coloration due to birefringence and the change thereof with the voltage application, it is theoretically difficult to obtain black and white display and the transmission light or reflection light of the liquid crystal cell is colored to thereby form a display on a colored background. In order to avoid such coloration, it has also been known to overlay another color-eliminating liquid crystal cell having a reverse twist direction of liquid crystal molecules on a STN type liquid crystal cell [refer, for example, to H. Watanabe et al., Society for Information Display 88 "Digest", PP. 416-419 (1988)]. In this case, however, since two liquid crystal cells are stacked, the cost is increased and the entire thickness and weight are also increased, and further, there is a drawback that displayed characters give an impression of floating since the distance between the polarization plate and the display liquid crystal layer is increased.
As a color-compensation plate for eliminating the coloration of the STN type liquid crystal display device, it has also been known a method of using a birefringence layer instead of the another liquid crystal cell [refer, for example, to H. Odai, et al. 1988, "International Display Research Conference", PP. 195-198 and Japanese Patent Application Laid-Open Sho 64-519 (1989)].
Such a method of using the birefringence layer is more useful industrially than the method of stacking two liquid crystal cells, since increase of the thickness and weight is not so much and the liquid crystal display device can be produced at a low cost. However, in the STN type liquid crystal display device color-compensated by using the birefringence layer there is a problem to be dissolved that the brightness and color are changed depending on the visual angle direction.
The present inventors have made a study for overcoming the foregoing problems and, as a result, have found that an excellent liquid crystal display device can be obtained by controlling the twist angle of liquid crystals in a liquid crystal cell, the maximum refractive index direction x, maximum refractive index n.sub.X and minimum refractive index n.sub.Y in the birefringence layer face parallel to the substrate surface and the refractive index n.sub.Z of the birefringence layer in the thickness direction each in a predetermined range.
That is, the object of the present invention is to provide a liquid crystal display device of excellent visual angle characteristic and capable of black and white display at a low cost.